Ontogeny of Feeding Performance in Herbivorous Mammals: a case study using sheep (Ovis aries)

Feeding is among the most important behaviors in vertebrate animals, directly involving and affecting morphology, function, behaviour, and ecology. While ontogenetic trajectories of feeding performance have been investigated in carnivorous and omnivorous mammals, herbivorous mammals remain understudied. Here we assess the ontogeny of feeding performance in the domestic sheep (Ovis aries) to establish a baseline for understanding the ontogeny of feeding performance in mammalian herbivores. We reconstructed six major jaw adductor muscles in an ontogenetic series of eight sheep skulls (newborn to 6.5 years) and estimated their bite force, jaw gape, and occlusal pressure. Bite force was calculated using physiological cross-sectional area estimates, jaw gape was derived from muscle strain limits, and occlusal pressure was measured as bite force relative to postcanine occlusal area. Results show that both bite force and jaw gape scale with positive allometry relative to skull size and age, with bite force increasing from ~100 N in neonates to ~650 N in adults, and jaw gape increasing from around 25° to 32°. Muscle orientations shift dorsoventrally through growth, likely optimizing efficiency of vertical biting at the expense of mediolateral and rostrocaudal movements. In contrast, occlusal pressure–potentially a more germane measure of feeding performance in herbivores–remains relatively constant throughout ontogeny, suggesting that feeding efficiency is constant through growth despite changes in bite force, jaw gape, and muscle efficiency. Thus, although juvenile sheep exhibit lower relative and absolute bite forces and jaw gapes, they may not be at a relative feeding performance disadvantage compared to adults if occlusal pressure is considered. More broadly, constant occlusal pressure through ontogeny may partly explain the absence of pronounced ontogenetic dietary shifts in mammalian herbivores